The historians in my area say that's the ancient American Indian burial ground. Which ancient American Indians used triangular-shaped burial grounds?
Going of the description indicating a Washington Township, I found the following: (Note see update below-this is the wrong county)
If my assumption on location is correct, the answer may be found on a Porter County genealogy website mentioning Native American Burial Grounds within the county. The Entry for Washington Township says the following (emphasis mine):
A. The 1882 History of Porter County, Indiana, states that (p. 178):
A village of about one hundred or more Pottawatomie Indians was situated near the present site of Prattville. Their burying-ground was located on what is now Harmond Beach's orchard.
An inspection of the 1876 plat map for Washington Township published by A. G. Hardesty, shows that Harmond Beach's orchard, and thus the Native American burial ground, is located in the northwest quarter of the northeast quarter of the southwest quarter of Section 21, Township 35 North, Range 5 West. No evidence of the Indian burial ground exists today.
Source Citations: Goodspeed, Weston A., and Charles Blanchard. 1882. Counties of Lake and Porter, Indiana: Historical and Biographical. Chicago, Illinois F. A. Battey and Company. 771 p. Hardesty, A. G. 1876. Illustrated Historical Atlas of Porter County, Indiana. Valparaiso, Indiana: A. G. Hardesty. 90 p.
I can't confirm the site location is the same you are referencing without more information, but this would indicate the region was previously inhabited by the Potawatomi. I agree with comments by @Mark C. Wallace however, that the triangular shape observed on the map image is most likely due to an artifact of farming practices or property boundaries, and is not likely in itself related to the burial grounds themselves.
It seems the location may be in Clark County and not Porter County. Both have Washington townships. The information I can dig up leads to a completely different and much older culture inhabiting the region.
The triangular section of woods is about 15 miles (24km) away from a site known as the Prather Site. This site is considered to be middle Mississippian Culture (c. 1200–1400CE) . This is also part of what is collectively known as the Mound Builders. The middle Mississippian era was before most of the modern tribal cultures we are familiar from early American History, but would be possibly ancestral to some of those groups:
Mississippian peoples were almost certainly ancestral to the majority of the American Indian nations living in this region in the historic era. The historic and modern day American Indian nations believed to have descended from the overarching Mississippian Culture include: the Alabama, Apalachee, Caddo, Chickasaw, Catawba, Choctaw, Muscogee Creek, Guale, Hitchiti, Ho-Chunk, Houma, Kansa, Missouria, Mobilian, Natchez, Osage, Quapaw, Seminole, Tunica-Biloxi, Yamasee, and Yuchi.
The proximity to the Prather site would lead me to think that, if there is a burial site within the triangular section of woods, this might be the connection. However I reiterate what I said before concerning the shape: I agree with comments by @Mark C. Wallace, that the triangular shape observed on the map image is most likely due to an artifact of farming practices or property boundaries, and is not likely in itself related to the burial grounds themselves.